CDMA mobile communication system and communication method

ABSTRACT

A radio communication system having a base station and a plurality of radio terminals, wherein each radio terminal having a transmission request transmits a reservation packet at arbitrary timing through a reservation channel in accordance with a CDMA scheme, and the base station assigns a traffic channel and a time slot to be used to each radio terminal requesting a reservation through a reply packet outputted onto a reply channel. On the reservation channel, a short spreading code corresponding to a matched filter is applied.

The present application is a continuation of application Ser. No.10/023,736, filed Dec. 21, 2001; which is a continuation of applicationSer. No. 09/511,769, filed Feb. 24, 2000, now U.S. Pat. No. 6,393,013;which is a continuation of application Ser. No. 08/690,819, filed Aug.1, 1996, now U.S. Pat. No. 6,269,088, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mobile communication system and acommunication method, and more particularly, to a reservation basedmobile communication system, mobile terminal equipment, andcommunication method to which code division multiple access (CDMA) isapplied.

2. Description of the Related Art

Conventionally, a mobile communication system which employs areservation based access control in a frequency division multiple access(FDMA) scheme is known, for example, as described in IEEE Transactionson Communications, Packet Switching in Radio Channels: “Part3-Pollingand (Dynamic) Split-Channel Reservation Multiple Access”, COM-24, 8,(1976), pp. 832-845 (hereinafter called “prior art publication 1”).

In the reservation based access control, each of mobile terminals havinga request for data transmission reserves a traffic channel to a basestation through a reservation packet. The base station, after schedulingtraffic channels and transmission timing (time slots) to be assigned tothese mobile terminals, notifies each of the mobile terminals oftransmission timing to be used on an assigned traffic channel through areply packet. According to this reservation based access control,collision of packets on the traffic channel can be basically avoided.

As another example of reservation based control type communicationsystem, for example, JP-A-6-311160, corresponding to U.S. patentapplication Ser. No. 08/230773 (hereinafter called “prior artpublication 2”) has proposed such a communication system based on a timedivision multiple access scheme.

However, in the mobile communication systems in which the reservationbased access control is applied to FDMA and TDMA schemes, as proposed byprior art publications 1 and 2, since respective mobile terminals sendreservation packets through a reservation channel asynchronously witheach other, a plurality of reservation packets can collide with a highpossibility. Thus, repetitive retransmission of reservation packetsobliged by the collision of packets constitutes a main cause ofdegrading the throughput of the entire communication system.

Meanwhile, as a standard for FPLMTS (Future Public Land MobileTelecommunication Systems), the adoption of the code division multipleaccess scheme is regarded as promising. A CDMA mobile communicationsystem has been proposed, for example, in JP-A-7-38496 corresponding toU.S. patent application Ser. No. 08/375679 (hereinafter called “priorart publication 3”). However, prior art publication 3 does not provideany useful information for solving the problem of a degraded throughputin the reservation based access control.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a mobilecommunication system and a communication method which employ areservation based access control to realize a high throughput.

It is another object of the present invention to provide CDMA mobileterminal equipment and base station which solve the problem of collisionof reservation packets to realize a high throughput.

To achieve the above objects, in a mobile communication system of thepresent invention, radio channels include a plurality of trafficchannels used for transmitting upward data packets directed from mobileterminals to a base station and for transmitting downward data packetsdirected from the base station to the mobile terminals, a reservationchannel used for transmitting reservation packets each indicative of atraffic channel assignment request from a mobile terminal to the basestation, and a reply channel used for transmitting reply packets eachindicative of a traffic channel through which data is transmitted andreceived from the base station to a mobile terminal, wherein thereservation, reply and traffic channels are applied with spread-spectrumin accordance with a CDMA scheme. The mobile communication system ischaracterized in that a mobile terminal having a request for datatransmission transmits a reservation packet onto the reservation channelat arbitrary timing, the base station specifies a traffic channel and atime slot to be used by the requesting mobile terminal by a reply packettransmitted through the reply channel, and each mobile terminaltransmits and receives a data packet in the time slot on the trafficchannel, both specified by the reply packet.

Describing in greater detail, each of the reservation, reply and trafficchannels is assigned a unique spreading code, for example, pseudonoise(PN). Particularly, the reservation channel is assigned a spreading codeshorter than those assigned to other reply and traffic channels. Thebase station relies on a matched filter to identify a plurality ofreservation packet signals having time-overlapped portions, transmittedfrom a plurality of mobile terminals, and to perform a receiving processon bit signals corresponding to each packet.

According to a preferred embodiment of the present invention, the basestation, upon receiving a reservation packet from a mobile terminal,assigns a time slot on a traffic channel in accordance with a schedulecontrol, and notifies each mobile terminal of the assignment resultthrough a reply packet.

Also, for regulating a total number of simultaneously communicatedpackets, the base station periodically transmits a busy tone signalindicative of a traffic situation, such that each mobile terminal havinga request for data transmission performs a reservation packettransmission control in accordance with the busy tone signal.Alternatively, the radio channels may be provided with a plurality ofreply channels so as to specify a reply channel for each mobile terminalto receive the busy tone signal therethrough.

According to the present invention, time slots are defined in thetraffic channels such that each mobile terminal transmits and receivesdata in a particular time slot specified by the base station. Thereservation channel, on the other hand, is not provided with time slots,so that each mobile terminal having a request for data transmissiontransmits a reservation packet at arbitrary timing, thus facilitatingthe operation of transmitting the reservation packet in each mobileterminal.

Also, each mobile terminal performs a spectrum spreading or multipliesthe reservation packet by a spreading code to generate a spread-spectrumreservation packet, where the spreading code has a period shorter thanthat applied to a data packet transmitted through a traffic channel,while the base station receives reservation packets using a matchedfilter.

In this case, even if two or more spread-spectrum control packets,modulated by the same spreading code, are partially overlapped on thetime axis, the matched filter can identify received packets, providedthat there is a timing deviation over one chip or more on the spreadingcode between the respective packets. Therefore, even if a plurality ofmobile terminals generate reservation packets individually at arbitrarytiming, a reception disabled condition caused by collision of thesepackets will occur with an extremely low possibility.

The foregoing and other objects, advantages, manner of operation andnovel features of the present invention will be understood from thefollowing detailed description when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary configuration of a mobile communicationnetwork to which the present invention is applied;

FIG. 2A is a diagram for explaining a protocol for a call set up processin a radio communication system according to the present invention;

FIG. 2B is a diagram for explaining a protocol for informationtransmission in the radio communication system according to the presentinvention;

FIG. 3 is a diagram for explaining a channel access control in aconventional radio communication system;

FIG. 4 is a diagram for explaining a channel access control in a radiocommunication system according to the present invention applying a CDMAscheme;

FIG. 5A illustrates a format for a reservation packet;

FIG. 5B illustrates a format for a reply packet;

FIG. 5C illustrates a format for an information transmission packet;

FIG. 6 is a block diagram illustrating the configuration of a basestation;

FIG. 7 is a block diagram illustrating the configuration of a CDMAtransceiver 50 in the base station;

FIG. 8A is a block diagram illustrating the configuration of a matchedfilter 70;

FIG. 8B is a diagram for explaining how the matched filter processesreceived reservation packets;

FIG. 9 is a block diagram illustrating the configuration of a packetseparation circuit 80;

FIG. 10 is a block diagram illustrating the configuration of a packetcontroller 90 in the base station;

FIG. 11 is a block diagram illustrating the configuration of a mobileterminal;

FIG. 12 is a block diagram illustrating the configuration of a CDMAtransceiver 110 in the mobile terminal;

FIG. 13 is a block diagram illustrating the configuration of a packetcontroller 130 in the mobile terminal; and

FIGS. 14A and 14B are diagrams for explaining a busy tone control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an exemplary configuration of a mobile communicationnetwork to which the present invention is applied.

The illustrated mobile communication network comprises a public network1 accommodating stationary terminals such as a telephone 3 or the like;and a mobile communication network 2 connected to the public network 1and accommodating a plurality of base stations 4 (4 a, 4 b, . . . ),wherein each base station 4 communicates with mobile terminals (radioterminals) 5 (5 a, 5 b, . . . ) located in its service area (cell)through radio channels 6. On the radio channel, a CDMA packettransmission is applied because of its suitability to communications ofmulti-media information in which data, sound and image signals aremixed.

FIG. 2A shows a protocol for a call set up process in the radiocommunication system according to the present invention.

The call set up process includes two different sequences of operations:one is a sequence of operations for initially allocating local ID's(local addresses) to mobile terminals in a service area, and the otheris a sequence of operations for allocating a link number to each mobileterminal for communicating with another destination terminal. The localID is an address number having a reduced length than that of a uniqueaddress previously assigned to each mobile terminal. The use of thislocal ID results in reducing the length of a packet. The link numberalso has a similar effect to the local ID.

A procedure of the call set up process is common to the above-mentionedsequences of operations for allocating the local ID's and for allocatingthe link numbers. Specifically, the procedure comprises the steps oftransmitting a control packet (reservation packet) 10 a for call set upfrom a terminal to a base station through a reservation channel 7;transmitting a control packet (reply packet) 11 a from the base stationto the terminal through a reply channel 8; and transmitting a call setup data packet 12 a from the base station to the terminal through atraffic channel.

Address information indicative of a source is set in the control packet10 a. Also, the address of a terminal required to receive the datapacket 12 a and a time slot on the traffic channel 9 in which the datapacket 12 a is to be received, are specified by the control packet 11 a,such that the terminal specified by this control packet 11 a receivesthe call set up data packet 12 a including location registrationinformation (local ID number) or link information (link number)transmitted by the base station in the specified time slot on thetraffic channel 9.

It should be noted that if the control packet 11 a has a sufficientlength, the location registration information or the link informationmay be transmitted through the control packet 11 a, instead of utilizingthe call set up data packet 12 a.

The reservation channel 7, reply channel 8, and traffic channels 9 aredistinguished by PN codes which are applied to spread-spectrum. Aplurality of traffic channels 9 can be formed by providing a pluralityof PN codes for transmitting data packets.

The base station is provided, for example, with a management table forindicating a slot using situation on each traffic channel such that thebase station schedules a slot for transmitting the data packet 12 a soas to minimize a waiting time of the terminal by referring to thismanagement table.

FIG. 2B shows a protocol for transmitting user information (hereinaftersimply called the “data”).

A terminal (transmitting terminal) having a request for datatransmission utilizes a PN code for the reservation channel 7 totransmit a control packet (reservation packet) 10 b for requesting theassignment of a slot in which a data packet is to be transmitted. Thebase station, in response to this request, utilizes a PN code for theresponse channel 8 to transmit a control packet (reply packet) 8 b tothe request transmitting terminal, thereby specifying a traffic channel9 i and a time slot to be used by the request transmitting terminal. Therequest transmitting terminal, upon receiving the reply packet 11 b,sends the data packet 12 b at the timing of a specified time slot on thetraffic channel 9 i.

The data packet 12 b is once received by the base station. The basestation confirms a destination address of the data packet, and utilizesthe PN code for the reply channel 8 to transmit a control packet 13 forspecifying a destination terminal (receiving terminal) as well as atraffic channel 9 j and a time slot with which the receiving terminal isto receive the data packet 12 b, when the receiving terminal is a mobileterminal located in the service area of the base station. Then, the basestation sends the received data packet 12 b from the requesttransmitting terminal as a data packet 14 in the specified time slot.The receiving terminal receives the data packet 14 transferred from thebase station in the specified time slot on the traffic channel 9 jspecified by the control packet 13.

According to the information transmission protocol described above,while a data transfer in the upward direction from a transmittingterminal to a base station requires a reservation packet, a datatransfer in the downward direction from the base station to a receivingterminal does not require the reservation packet.

The base station provides each mobile terminal with reference timing indata packet transmission/reception operations using a pilot signaltransmitted through a pilot channel in parallel with the transmission ofthe data packet 14. Since each mobile terminal can receive the datapacket 14 and the pilot signal transmitted from the base station withthe same delay time, the mobile terminal can readily accomplishsynchronization acquisition, when receiving the data packet 14, bydetermining the timing of a receiving time slot based on the pilotsignal.

FIG. 3 shows a reservation based access control in a conventional FDMAradio communication system.

As described above in connection with FIG. 2A, the reservation basedaccess control is a control method in which a reservation packet is sentprior to the transmission of a data packet, and the data packet istransmitted after the reservation is established. For this control, thereservation channel 7 and the reply channel 8 are provided in additionto the traffic channels 9. The channels may be divided in accordancewith the time division multiple access (refer to the prior art 2) otherthan the frequency division multiple access (refer to the prior art 1)shown in FIG. 3.

In FIG. 3, the abscissa represents the time axis 21. When a radioterminal transmits a reservation packet to a base station through thereservation channel 7, the base station schedules time slots on thetraffic channels, and transmits a reply packet indicative of areservation result to the radio terminal through the reply channel 8.

In the conventional reservation based access control, if a plurality ofradio terminals transmit reservation packets onto the reservationchannel 7 at a time, the reservation packets may collide with each otherand collapse, as indicated by 22 a, 22 b in FIG. 3, with the result thatthe base station cannot receive the reservation packets. Each radioterminal determines that its reservation packet would have collided withany other reservation packet on the reservation channel if a replypacket destined thereto has not been returned in a predetermined timeperiod after the radio terminal had sent the reservation packet. In thisevent, the radio terminal again transmits the reservation packet(indicated by 23 a, 23 b). Thus, the throughput in a radio communicationsystem employing the conventional reservation based access control islimited depending on the collision of reservation packets as describedabove.

FIG. 4 shows an access control in a reservation based CDMA radiocommunication system according to the present invention.

The present invention applies CDMA packet transmission to a reservationchannel to allow a plurality of radio terminals to transmit reservationpackets individually at arbitrary timing.

In a reservation channel 7 illustrated in FIG. 4, the ordinaterepresents transmitting terminals 25. FIG. 4 represents a situation inwhich the transmitting terminals 25 have transmitted reservation packetspartially overlapped on the time axis 21.

In the CDMA scheme, the spread-spectrum is applied by replacing eachsymbol (bit “1” and “0”) in transmitted data with a spreading code(orthogonal code or PN code) composed of a plurality of chips havingunique patterns. For example, in a direct sequence spread-spectrum, aplurality of transmitting terminals modulate transmission data using thesame PN (pseudonoise) sequence, and transmit the spread-spectrum data atthe same carrier frequency. In this event, if there is a time deviationof one chip or more in transmission timing between respective symbols indata, the receiving side can individually identify each of transmitteddata.

If a plurality of reservation packets are transmitted at completely thesame timing, the packets will collide, whereby destinations will fail toreceive the reservation packets. However, generally, such transmissionof a plurality of reservation packets at completely the sametransmission timing is rather a rare case. In the spread-spectrum, evenif two packets are time-overlapped, the collision is avoided when thesepackets are deviated in timing by a time equal to or longer than onechip, as indicated by 26 a, 26 b in FIG. 4, thus eliminating the need toretransmit the reservation packets. It will be appreciated that thereservation based control scheme according to the present inventionsignificantly improves the throughput compared with the conventionalreservation based communication system.

In the present invention, each radio terminal having a request for datatransmission transmits a reservation packet at arbitrary timing on thereservation channel, and sends a data packet in a time slot on a trafficchannel, both specified by a reply packet received through the replychannel.

The data packet is transmitted in units of time slot in principle. Whentransmission data is so long that a plurality of time slots are requiredfor the transmission, the data is divided into a plurality of datapackets, and a time slot is reserved for each data packet. However, forreducing overhead due to the reservation process, a plurality of timeslots may be reserved by a single reservation packet such that a basestation, in response to the reservation packet, assigns a plurality ofcontinuous or intermittent time slots to a transmitting terminal by asingle reply packet or a plurality of reply packets generated forrespective time slots.

While the present invention allows the mobile terminals to transmitreservation packets at arbitrary timing, the mobile terminals musttransmit and receive a reply packet and a data packet in synchronismwith a time slot having a previously defined constant length.

As illustrated in FIG. 4, the reply channel 8 and the respective trafficchannels 9 are divided into time slots respectively having a fixedlength, and a pilot signal is used to match the timing, thusfacilitating fast synchronization of spreading codes between each radioterminal and a base station. More specifically, the base station spreadsthe pilot signal (reference signal) with a spreading code (PN sequence)having a suitable period, and continuously transmits the spread-spectrumpilot signal on a common channel (pilot channel). Each radio terminalgenerates a synchronization signal based on the pilot channel despreadfrom the spread-spectrum pilot signal with a PN sequence unique to thepilot channel, and sets a time slot in synchronism with the base stationon the reply channel and on each traffic channel.

It should be noted that since the pilot signal is intended for thesynchronization of the spreading codes, the pilot signal may include anycontents. Thus, for transmitting the pilot signal, the reply channel,for example, may be utilized instead of using the dedicated pilotchannel.

FIGS. 5A-5C illustrate formats for the packets used in the mobilecommunication system according to the present invention.

The reservation packet, as illustrated in FIG. 5A, is composed of apreamble 31 a for synchronization acquisition; a type of reservation 432b indicative of the type of the packet (identification code foridentifying a location registration packet, a link securing packet, or atraffic channel reserving packet); a source address 33 (using a local IDif the location has been registered); a destination address 34 (using alink number if a link has been secured); a number 35 of reservationdesired transmission packets (time slots); and a CRC (Cyclic RedundancyCheck) code 36 a serving as an error detection code, arranged in thisorder from the beginning. The number 35 of transmission packets is notrequired in the call set up process for location registration or linksecuring.

The reply packet, as illustrated in FIG. 5B, is composed of a sourceaddress 34; a type of reply 32 b indicative of the type of the packet(for identifying a location registration packet, a link securing packet,an upward direction information transmitting packet or a downwarddirection information transmitting packet); a PN type 37 indicative of aspreading code of a traffic channel to be sued; timing information 38indicative of assigned transmission timing (time slot); and a CRC code36 b, arranged in this order from the beginning.

It should be noted that in the present invention, the reply packet doesnot require a preamble. This is because each radio terminal can acquireeach reply packet by receiving the pilot signal and establishing thesynchronization of each time slot on the reply channel based on thepilot signal, as described above.

The data packet for transmitting information, as illustrated in FIG. 5C,is composed of a preamble 31 b; a type of packet (for identifying alocation registration packet, a link securing packet, an upwardinformation transmitting packet, or a downward information transmittingpacket) 32 c; a source address 33 (using a local ID if the location hasbeen registered); a destination address 34 (using a link number if alink has been secured); data 39 (a PN code for the informationtransmitting channel or the reply channel, transmission or receptiontiming, and transmission information); and a CRC code 36 c, arranged inthis order from the beginning.

Since the reply channel and the traffic channel for transmittinginformation are respectively divided into packets, it is desirable thatthe sizes of respective packets be unified to a fixed length even if thetypes of packets are different. For this purpose, dummy bits may beinserted in a front portion of each packet so as to adjust the beginningposition of respective fields subsequent thereto. In the downward datapacket, the preamble 31 b may be omitted as is the case of the replypacket.

FIG. 6 illustrates a schematic configuration of the base station 4.

The base station 4 comprises an antenna 41; a CDMA transceiver 50; apacket controller 90; a BSC interface 42 connected to a controller (BSC43) intervening between the base station 4 and the mobile communicationnetwork 2.

FIG. 7 illustrates in detail the configuration of the CDMA transceiver50 in the base station. The CDMA transceiver 50 comprises receivingradio module 52 and a transmitting radio module 53 for modulating anddemodulating a baseband signal as well as for transmitting and receivingsignals at radio frequencies.

Referring specifically to FIG. 7, a control packet (reply packet) signaltransmitted from a base station to a radio terminal is inputted to anencoder 58 a through a reply channel signal line 45 a, and is subjectedto encoding for error correction using, for example, a convolutionalcode or the like. The encoded reply packet signal is multiplied by anorthogonal code for the reply channel outputted from an orthogonal codegenerator 59 in a multiplier 56 a to generate a spread-spectrum replypacket signal which is then inputted to an adder 60.

Similarly to the reply packet signal, data packet signals outputted to aplurality of signal lines 45 b respectively corresponding to trafficchannels are encoded in the encoder 58 b, and multiplied by orthogonalcodes corresponding to respective traffic channels in a multiplier 56 bto generate spread-spectrum data packet signals which are then suppliedto the adder 60. A pilot signal outputted to a signal line 45 c islikewise encoded in an encoder 58 c, multiplied by an orthogonal codeunique to the pilot channel in a multiplier 56 c to generate aspread-spectrum pilot signal which is then supplied to the adder 60.

The output of the adder 60 is multiplied by a PN code (long code) uniqueto each base station outputted from a PN generator 57 a in a multiplier56 to generate a spread-spectrum signal which is subsequently suppliedto the transmitting radio module 53.

On the other hand, a received signal processed by the receiving radiomodule 52 is inputted to a matched filter 70 a for the reservationchannel and to a plurality of matched filters 70 b-70 b′ respectivelycorresponding to traffic channels.

The matched filter 70 a despreads the received signal with a PN codeunique to the reservation channel. The despread signal is separated intoa plurality of bit data trains 89 each for a corresponding reservationpacket in a packet separation circuit 80. In this case, as describedlater with reference to FIGS. 8 and 9, if the period of a PN sequenceapplied to the despreading process is selected to be equal to the numberof taps of the matched filter, the outputs of the matched filter can beused as despread results without further processing, thus realizing fastsynchronization. Each bit data train for a corresponding reservationpacket, separated from other bit data trains in the packet separationcircuit 80, is subjected to a decoding process accompanied by errorcorrection, for example, such as Viterbi decoding or the like in adecoder 55 a, and subsequently supplied to the packet controller 90.

The matched filters 70 b-70 b′ are provided for acquiring the initialsynchronization of PN sequences of received signal son the respectivetraffic channels. Once the synchronization is acquired, each of the PNgenerators 57 b, 57 b′ generates a PN sequence for each channel insynchronism with the acquired PN sequence. The received signal ismultiplied by PN sequences corresponding to respective channelsgenerated by the PN generators 57, 57 b in multipliers 56, 56′ to bedespread. The despread signals are accumulated for every one symbollength in accumulators 54, 54′. The accumulated results are decoded bydecoders 55, 55′ and subsequently supplied to the packet controller 90as data packet signals for the respective traffic channels.

FIG. 8A illustrates the principle of the matched filter 70 a. Thematched filter 70 is composed of a plurality of cascaded delay elements71 each having a delay time T equal to a chip width of a PN sequence; aplurality of taps arranged on the input side of the delay element at thefirst stage and on the output side of the respective delay elements; anda plurality of coefficient multipliers 72, one in each tap. The matchedfilter 70 a is configured such that received signals inputted at everychip time propagate from one tap to the next in the delay time T.

In the matched filter 70 a for the reservation channel, the delay timeof each delay element 71 is equal to the chip width of a PN sequence forthe reservation channel, and the number of taps is equal to the numberof chips included in one period of the PN sequence, such that aone-period portion of the PN sequence simultaneously appears at theplurality of taps at the time the top chip of an inputted signal reachesthe rightmost tap. Therefore, respective chip values (“1” or “−1”) ofthe PN sequence a1-an for the reservation channel are previously set inthe respective coefficient multipliers 72 as coefficients, and a totalsum of the results of multiplications of respective tap outputs by therespective coefficients is calculated by an accumulator 73. If theaccumulation result is outputted as a correlation value between thereceived signal and the PN sequence for the reservation channel, thesynchronization is acquired at the time the correlation value changingfor every chip time presents a peak value. Also, the output value of theaccumulator 73 at this time indicates a demodulated value generated bydespeading the received signal.

In the present invention, the number of taps of the matched filter 70 ais made equal to a spreading code length so that the output 79 a of thematched filter 70 a contains information (symbol code) of a one-bitportion of the reservation packet. Also, a short code type PN sequencehaving a less number of chips is applied as a spreading code for thereservation channel to reduce the number of taps required to the matchedfilter, thus facilitating the synchronization acquisition.

FIG. 8B illustrates an output signal of the matched filter 70 a which isgenerated when two reservation packets A, B are partially overlapped onthe time axis.

The output signal 79 a of the matched filter 70 a includes a pluralityof positive peak values (indicative of a code bit “1”) and a pluralityof negative peak values (indicative of a code bit “0”) generated by theaccumulator 70 a. Peak values equal to or more than a predeterminedthreshold are detected from the output of the matched filter 70 a andgrouped into groups of signals appearing at a time interval matchingwith the PN sequence period from the respective start points at whichthe first peak values are detected (synchronization acquisition time),thereby making it possible to identify a bit data train 78 belonging tothe reservation packet A and a bit data train 76 belonging to thereservation packet B.

In the illustrated example, the peak value 76-1 appearing first isdefined as the start point, and signal values(“1” or “−1”) 76-2, 76-3,76-4, . . . subsequently appearing at a time interval equal to the PNperiod 75 are extracted from the output of the matched filter 70 a toreproduce the bit data train 76 constituting the reservation packet A.Also, a peak value 77-1 appearing asynchronously with the bit data train76 is defined as the start point, and signal values (“1” or “−1”) 77-2,77-3, 77-4, . . . are extracted at a time interval equal to the PNperiod 75 are extracted from the output of the matched filter 70 a toreproduce a bit data train 77 which constitutes the reservation packetB. By applying a similar principle, even if three or more reservationpackets are transmitted in a time-overlapped condition, bit signals foreach packet can be identified as long as a phase deviation over one chipor more exists between the respective packets.

FIG. 9 illustrates an exemplary configuration of the packet separationcircuit 80.

The output signal 79 a of the matched filter 70 a is inputted to anabsolute value circuit (ABS) 81, the output of which is compared with apredetermined threshold outputted from a threshold circuit 82 by acomparator 83 a. When the output of the absolute value circuit 81 islarger than the threshold, the output of the comparator 82 is turned ON(“1” state) and inputted to an AND circuit 84 a. Since the AND circuit84 a is also supplied, as other input signals, with inverted signalswhich are initially OFF (“0” state), the AND circuit 84 is opened by theON output from the comparator 83 a, whereby its output signal is turnedON (“1” state). The ON output from the AND circuit 84A is inputted toAND circuits 84 b and 84 d.

The AND circuit 84 b is also supplied at the other input terminalthereof with an inverted version of an output signal from a timer 85 a.In an initial state, the output of the timer 85 a is in OFF state (“0”state), so that the output of the AND circuit 84 b is also turned ON atthe time the output of the AND circuit 84 a is turned ON. The ON outputof the AND circuit 84 b is inputted to a timing register 86 a as anenable signal, whereby the timing register 86 a is set at a valuerecorded on a counter 87 which performs a counting operation at aninterval equal to the chip period of the PN code and returns to aninitial value at an interval equal to the symbol length. The counter 87outputs a value which indicates a chip position at the timing at whichthe synchronization is acquired, as previously described with referenceto FIG. 8B.

The ON output of the AND circuit 84 b causes a timer 85 a to start forcontrolling the other input terminals of the AND circuits 84 b and 84 d.The timer 85 a maintains its output in ON state for a time periodcorresponding to one reservation packet. This permits the AND gate 85 dto remain open and the AND gate 84 b to remain close until a time set inthe timer 85 a expires, thus preventing any other counted value frombeing set in the first timing register 86 a.

If the next peak value is outputted from the matched filter 70 a beforethe time set in the timer 85 a expires, the ON output from the ANDcircuit 84 a is inputted to an enable terminal of a second timingregister 86 b through a pair of AND circuits 84 d and 84 d′ which remainopen. As a result, the output value of the counter 87 is set in thesecond register 86 b. At this time, a timer 85 b cooperating with thesecond timing register 86 b is started and performs a similar operationto that of the timer 85 a to prohibit any other value from being set inthe second timing register 86 until a one-packet period has elapsed andto open a pair of AND gates at the next stage so as to input thesubsequently generated enable signal to a third timing register 86 c.

In this embodiment, since the packet separation circuit 80 is providedwith four timing registers 86 a-86 d, the synchronization acquisitiontiming is stored for four reservation packets, determined by the orderof generation, within a plurality of reservation packets generated in atime-overlapped condition by repeating the foregoing operations in asimilar manner.

The value of the synchronization acquisition timing set in the timingregister 86 a is compared with an output value of the counter 87 in acomparator 83 b. Every time the counted value is coincident with thesynchronization acquisition timing value set in the timing register 86a, the output of the comparator 83 b is turned ON.

The ON output of the comparator 83 b is inputted to an enable terminalof a data register 87 a through the AND circuit 84 c which remains openwhile the timer 85 a is in ON state. As a result, the data register 87 ais supplied with the output of the matched filter 80 a at thesynchronization acquisition timing. The remaining timing registers 86b-86 d also operate in a manner similar to the foregoing to store theoutputs of the matched filter 70 a for respective reservation packets indata registers 87 b-87 d, respectively.

Since the data registers 87 a-87 d are supplied with data in accordancewith the synchronization acquisition timing of the respectivereservation packets, the contents of these data registers 87 a-87 d aretransferred to output registers 88 a-88 d, respectively, in synchronismwith a clock having a bit period generated by a clock generator 88, anddata indicative of the contents of the respective reservation packetsare transferred to the decoder 55 a illustrated in FIG. 7 from theoutput registers 88 a-88 d.

FIG. 10 illustrates an exemplary configuration of the packet controller90 in the base station 4.

Received data from the reservation channel (the contents of areservation packet) is inputted to a digital signal processor (DSP) 91,and is processed by a reservation packet processing routine 92 of theDSP 91. Subsequently, an assignment of a traffic channel and a time slot(scheduling) is performed by an upward schedule control routine 93.

A traffic channel (PN type) and a time slot (timing information)determined by the upward schedule control routine 93 is transferred to areply packet constructing unit 97 together with a source address of areservation packet to which a reply packet is destined. The reply packetconstructing unit 97 generates a reply packet including the aboveinformation and transmits it to the reply channel signal line 45 a. Inthis way, the operation for transmitting an upward data packet from eachmobile terminal can be controlled in accordance with the scheduling ofthe base station.

Received data from respective traffic channels are inputted to receptionprocessing units 96 b, 96 b′ arranged in correspondence to therespective traffic channels through signal lines 44 b, 44 b′, andtransferred to the BSC interface 42 through signal lines 46 as receiveddata packets.

On the other hand, a downward data packets outputted from the BSCinterface 42 to signal lines 47, after temporarily stored intransmission buffers 99, 99′, are transmitted under the control of aschedule executed by a downward schedule control routine 95 of the DSP91. More specifically, in accordance with a downward schedule, a replypacket constructed by the reply packet constructing unit 97 is firstsent from the reply channel, and subsequently data packets generated bythe traffic packet constructing units 98 a, 98 a′ are sent inpredetermined time slots on traffic channels determined by the downwardschedule.

In this embodiment, for restraining mobile terminals from issuingreservation packets when the traffic channels remain busy, a busy tonevalue calculation routine 94 of the DSP 91 generates busy toneinformation in accordance with the number of reservation packetsreceived through the reservation channel and traffic channel utilizationstate information known to the upward schedule control routine 93, andnotifies the busy tone information to the respective mobile terminalsthrough the reply channel 45 a.

FIG. 11 illustrates the configuration of the radio terminal 5.

The radio terminal 5 is composed of an antenna 100; a CDMA transceiver110 connected to the antenna 100; a packet controller 130 connected tothe CDMA transceiver 110; and a data processing unit connected to thepacket controller 130.

The data processing unit comprises a microprocessor (MPU) 101; a memory102 for storing data and programs; and a plurality of input/outputdevices connected to an internal bus through an I/O interface 103. Theinput/output devices may comprise, for example, a camera 104 a, aspeaker 104 b, a display 104 c, a keyboard 104, and so on.

FIG. 12 illustrates in detail the configuration of the CDMA transceiver110 in the radio terminal.

The CDMA transceiver 110 comprises a receiving radio module 112 and atransmitting radio module 113. These modules are responsible formodulation or demodulation of a baseband signal and a receiving processor a transmitting process at radio frequencies.

In a transmitter circuit, a reservation packet signal outputted to areservation channel signal line 106 a is encoded for error correction inan encoder 120 a, and then multiplied by a unique PN sequence (shortcode) generated from a PN generator 121 a in a multiplier 114 a togenerate a spread-spectrum reservation packet signal which is sent tothe transmitting radio module 113.

On the other hand, a data packet outputted to a traffic channel signalline 106 b is encoded for error correction in an encoder 120 b, andmultiplied by a PN sequence (long code) generated by a PN generator 121b in a multiplier 114 b to generate a spread-spectrum data packet whichis sent to the transmitting radio module 113. The spread-spectrum forthe data packet is performed using a PN sequence specified by a basestation, which is identified by a control signal outputted onto a signalline 106 c by a packet controller 130 and in synchronism with referencetiming 105 c provided from a PN generator 119 in a receiver circuit.

In the receiver circuit, a received signal outputted from the receivingradio module 112 is inputted to a multiplier 114 c which multiplies thereceived signal by a PN code unique to the base station generated by thePN generator 119 to despread the received signal. The output of themultiplier 114 c is parallelly inputted to multipliers 114 d, 114 e and114 f respectively for the reply channel, traffic channels and pilotchannel, and multiplied by orthogonal codes unique to the respectivechannels generated by an orthogonal code generator 117.

On a reply channel line 105 a and a traffic channel line 105 b, outputsignals from the multipliers 114 d, 114 e are inputted to accumulators115 d, 115 e, respectively, to produce accumulated values for eachsymbol length for despreading the output signals from the multipliers114 d, 114 e. Output signals of the respective accumulators 115 d, 115 eare inputted to decoders 116 d, 116 e, respectively, for errorcorrection, and then transferred to the packet controller 130 throughsignal lines 105 d, 105 e, respectively.

On a pilot channel line 122, a pilot signal outputted from anaccumulator 115 f is inputted to a DLL (Delay Locked Loop) circuit 118for tracking of synchronization. The PN generator 119 is forced togenerate a PN sequence in synchronism with the output of the DLL circuit118. It should be noted that the decoders 116 d, 116 e on the replychannel line 105 a and the traffic channel line 105 b are operated insynchronism with the pilot signal outputted from the accumulator 115 f.

FIG. 13 illustrates an exemplary configuration of the packet controller130 in the radio terminal.

Received data through the reply channel appearing on the signal line 105a is inputted to a DSP 131 and precessed by a monitoring routine 132.The contents of the reply packet is supplied to an upward schedulecontrol routine 134 and to a downward schedule control routine 135,while a busy tone signal received through the reply channel is suppliedto a busy tone calculation routine 133.

Received data through a traffic channel appearing on the signal line 105b is received by a reception processing circuit 136 which is controlledby a control signal from the downward schedule control routine 135 and areference timing signal 105 c, and received data in a particular timeslot specified by a base station through a reply packet is outputtedonto a signal line 107 as receiving information.

On the other hand, transmission data from the radio terminal, aftertemporarily stored in a transmission buffer 138, is fetched by a trafficpacket constructing unit 139 in accordance with an instruction from theupward schedule control routine 134, and is sent onto the trafficchannel signal line 106 b as a data packet.

When a reply packet is received from a base station, the upward schedulecontrol routine 134 generates a signal 106 for specifying a trafficchannel (PN sequence) to which a traffic packet is to be sent, andissues a data packet sending instruction to the traffic packetconstructing unit 139 at timing of a time slot specified by the basestation. The traffic packet constructing unit 139, upon receiving thedata packet sending instruction from the control routine 134, readstransmission data from the transmission buffer 138, and sends the datapacket illustrated in FIG. 5C onto the traffic channel signal line 106 bat predetermined output timing determined based on the reference timingsignal 105 c.

The busy tone value calculation routine 133 calculates a busy tone valueindicative of a traffic situation from a busy tone signal receivedthrough the reply channel, and notifies the busy tone value to theupward schedule control routine 134.

The upward schedule control routine 134 controls the generation ofreservation packets in accordance with the traffic situation. Forexample, if the busy tone signal does not indicate to restrain datatransmission with transmission data being accumulated in thetransmission buffer, the reservation packet constructing unit 137 isstarted at arbitrary timing to transmit a reservation packet to thereservation channel signal line 106 a. Conversely, if the busy tonesignal indicates to restrain data transmission, the transmission ofreservation packets is restrained until the traffic situation improves.

As described above, in this embodiment, the CDMA scheme is applied tothe reservation channel to reduce the possibility of retransmission ofreservation packets due to collision of the reservation packets even ifrespective mobile terminals transmit the reservation packets atarbitrary timing. Moreover, the busy tone control is added to restrainthe transmission of new packets from mobile terminals when the trafficchannels or the reservation channel is in an overload condition.

The CDMA has a problem that when a plurality of packets are generated ina time-overlapped condition, the packet signals mutually affect asnoise, so that if a large number of packets are simultaneouslygenerated, the receiver side cannot identify them because all packetsignals are buried in noise. As described above, in the mobilecommunication system of the present invention comprising a reservationchannel, a reply channel and a plurality of traffic channels, the totalnumber of reply packets and data packets can be controlled by thescheduling function of the base station, whereas the base station cannotdirectly control reservation packets since they are issued autonomouslyfrom respective mobile terminals.

As described above, a method which allows each radio terminal toautonomously control the transmission of a reservation packet withreference to the busy tone signal from the base station is effective inavoiding concentrated reservation packets to smoothly control thetransmission in each terminal.

While the busy tone signal may be transmitted through a channeldedicated thereto, empty time zones appearing periodically on the replychannel may also be utilized.

The reply channel, as shown in FIG. 2, is divided into time slots eachhaving a length corresponding to the length of a data packet on thetraffic channel based on the pilot signal. Since the reply packetincludes a smaller amount of information, its length can be made shorterthan the data packet. For example, assuming that the time slot length(data packet length) is 512 bits and the reply packet length is 42 bits,12 reply packets can be transmitted through the reply channel during onetime slot period on the traffic channel, with a 8-bit empty time zoneremaining at the end of the time slot. It is therefore possible toutilize the available empty time zone in the time slot to periodicallytransmit the busy tone signal through the reply channel.

Next, a reservation packet restraining method using the busy tone signaltransmitted in an empty time zone on the reply channel will be describedwith reference to FIGS. 14A, 14B.

In FIG. 14B, “t−1”, “t” and “t+1” designate time slot numbers on thereply channel, and a pulse waveform represents the busy tone signal 143.The busy tone signal 143 is periodically transmitted utilizing an emptytime zone left in each time slot on the reply channel.

FIG. 14A shows a relationship between a total amount of packets sent outby radio terminals in each time slot and a number T of allowed packetswhich can be transmitted in a time-overlapped condition. An area 148indicates an amount of reservation packets sent in the time slot “t−1”and an area 149 indicates an amount of data packets sent in the timeslot “t−1”.

In the following, the busy tone signal generated by the base station inthe time slot “t−1” will be described, assuming that a number oftransmitted data packets during the time slot “t” is I(t), a number oftransmitted reservation packets is R(t), a number of transmissionrequested reservation packets is R(t)′, and a transmission probabilityof reservation packets is P(t). Further, R(t)′ and R(t) are defined tobe numbers of reservation packets when the length of the reservationpacket is normalized by the length of the data packet.

First, assume the following equation (1):1R(t)′=R(t−1)P(t−1)  (1).

Assuming that the number R(t)′ of transmission requested reservationpackets possessed by all radio terminals in the service area of a basestation in the time slot “t” is equal to a number R(t−1)′ oftransmission requested reservation packets in the previous time slot“t−1”, the equation (1) is derived by substituting a number R(t−1) ofreservation packets actually received by the base station as the valueof R(t−1)′. To the base station, the number l(t) of data packets in thetime slot “t” is known from previously received reservation packets andthe result of scheduling the traffic channels for received data packetsfrom other base stations.

Thus, the value of R(t)′ is estimated from the equation (1), and when atotal amount of the number R(t)′ of transmission requested reservationpackets and the number I(t) of data packets in the time slot “t” exceedsa tolerable value T as shown by the following equation (2), thetransmission of reservation packets is restrained by the busy tonesignal:I(t)+R(t)′≧T  (2)

In this event, the transmission of reservation packets is controlled bythe busy tone signal such that the transmission probability P(t) ofreservation packets from radio terminals in the service area isrestrained by a traffic amount on the traffic channels, as shown by thefollowing equation (3), thereby making the sum of the number ofreservation packets and the number of data packets substantially equalto the tolerable value T. Since the number of reservation packetsactually transmitted from radio terminals is determined from theprobability, it is desirable that the tolerable value T be set at aslightly lower level in order to allow for a certain margin.$\begin{matrix}{{P(t)} = \frac{\left\{ {T - {I(t)}} \right\}}{{R(t)}^{\prime}}} & (3)\end{matrix}$

On the other hand, if a total amount of packets estimated in the timeslot “t” is in a relationship expressed by the following equation (4),the transmission of reservation packets is controlled by the busy tonesignal such that the transmission probability P(t) follows the equation(5), thus allowing all radio terminals to freely transmit reservationpackets.I(t)+R(t)′<T  (4)P(t)=1.0  (5)

The base station may notify respective radio terminals of informationindicative of the transmission probability expressed by the equation (3)or (4) as the busy tone signal 143 in the time slot “t−1”.

As will be apparent from the foregoing description, the presentinvention applies CDMA to a reservation based packet access control typemobile communication system to reduce the possibility of retransmissionof reservation packets due to their collision, even if each mobileterminal is allowed to transmit a reservation packet at its arbitrarytiming, to improve the throughput.

According to the present invention, for example, a short spreading codeis applied to a reservation packet, and the synchronization is acquiredon the base station side using a matched filter, so that even if aplurality of mobile terminals transmit reservation packetsasynchronously to each other, the base station can identify therespective reservation packets at a high speed. Also, a reduced localaddress (own address) shorter than an original address number or a linknumber (destination address) is used for terminal address informationset to each packet, so that the transmission efficiency can be improved.Further, when each terminal is allowed to control the transmission ofreservation packets in accordance with a busy tone signal from a basestation, it is possible to avoid an excessive amount of reservationpackets simultaneously communicated on a channel, thus ensuring afavorable communication environment.

It is to be understood that the above-described embodiments are merelyillustrative of the principles of the invention and that may variationsmay be devised by those skilled in the art without departing from thespirit and scope of the invention. It is therefore intended that suchvariations be included within the scope of the claims.

1. A communication method in a radio communication system, wherein abase station and a plurality of radio terminals communicate in radiochannels, said method comprising the steps of: transmitting areservation using Code Division Multiple Access (CDMA) from one of saidplurality of radio terminals requesting data transmission; receiving areply from the base station, corresponding to the reservation, in theone radio terminal; and transmitting a data packet from the one radioterminal in response to the reply, wherein a single data packet istransmitted in response to a single reply packet.
 2. The methodaccording to claim 1, wherein a single reservation is transmitted for asingle data packet.
 3. The method according to claim 2, wherein a singledata packet has a constant length not longer than a time slot.
 4. Themethod according to claim 3, wherein a call setup process between theone radio terminal and the base station is performed in advance, andwherein the one radio terminal transmits a plurality of reservations anddata packets after the call setup process.